1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a CMOS image sensor and method for fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for enhancing light-condensing and photosensitivity characteristics.
2. Discussion of the Related Art
An image sensor is a semiconductor device that converts an optic image to an electric signal. In a charge-coupled device (CCD) image sensor, a plurality of metal-oxide-semiconductor (MOS) capacitors are arranged close to one another to transfer and store electrical charge carriers. In a complementary MOS (CMOS) image sensor, a plurality of MOS transistors, corresponding to the number of pixels, are fabricated by CMOS technology using a control circuit and a signal processing circuit as peripheral circuits and a switching system of sequential detecting outputs using the MOS transistors.
The CCD has a complicated drive system, high power consumption, a complicated fabricating process including too many steps, and difficulty in being implemented into one-chip due to the difficulty in implementing a signal processing circuit within a CCD chip. To overcome these disadvantages, sub-micron CMOS fabrication has been developed. That is, a CMOS image sensor enables the one-chip implementation with several signal processing circuits. The CMOS image sensor reproduces an image using a photodiode and MOS transistors within a unit pixel and detects signals sequentially by a switching system. Thus, the CMOS image sensor adopts CMOS fabrication technology and its fabrication process, which requires about twenty masks and is simpler than the CCD process that needs at least thirty to forty masks.
Referring to FIG. 1, a unit pixel of a conventional CMOS image sensor includes a photodiode PD for receiving light to generate photocharges, a transfer transistor Tx for transferring the photocharges generated from the photodiode PD to a floating diffusion (FD) region, a reset transistor Rx for setting a potential of the floating diffusion region to a specific value and resetting the floating diffusion region by discharging electric charges, a drive transistor Dx acting as a source-follower buffer amplifier, and a select transistor Sx acting a switching device to enable addressing. A load transistor is provided outside the unit pixel to read an output signal.
FIG. 2 illustrates a unit pixel of a conventional CMOS image sensor, in which elements associated with light-condensing and focusing are shown.
Referring to FIG. 2, a field oxide layer (not shown) is formed on a semiconductor substrate 11 divided into a sensor part and a peripheral drive part to define an active area. A plurality of photodiodes 12 and transistors 13 are formed over the semiconductor substrate 11. A first insulating interlayer 14 is formed over the semiconductor substrate 11, including the photodiodes 12 and transistors 13. A plurality of metal lines M1-M4 are formed over the first insulating interlayer 14 to configure a unit pixel. The metal lines M1-M4 are arranged to pass incident light, which strikes the corresponding photodiode 12. Second, third, and fourth insulating interlayers 15, 16, and 17 and a planarizing layer 18 are formed between the metal lines M1-M4 for electrical insulation. An RGB color filter layer 19 for color image implementation is formed on the planarizing layer 18 in the sensor part. A plurality of domed microlenses 20 of photoresist are formed on the color filter layer 19 to concentrate incident light on the photodiode 12. Each microlens 20 is formed by coating the photoresist, patterning the photoresist to cover the corresponding photodiode 12, and carrying out a reflow process on the patterned photoresist to obtain a specific curvature.
When a device is highly integrated, however, the metal lines are provided to different layers, which increases the total thickness of the insulating interlayers and the distance between the microlenses 20 and photodiodes 12. Therefore, it is difficult for the microlenses 20 to properly focus the light onto the corresponding photodiode. In particular, although there are only two tiers to the metal lines M1 and M2 in the sensing part, the second, third, and fourth insulating interlayers 15, 16, and 17 are stacked. Hence, the thick layers that substantially exist between the microlens 20 and the photodiode 12 weakens incident light to degrade image quality. In addition, deviating from an incident angle due to the distance between the microlens 20 and the photodiode 12, incoming light enters a neighboring pixel to bring about color interference of crosstalk. Hence, the image quality is degraded as well.
To overcome the above problems, the thickness of each of the insulating interlayers over the photodiode 12 can be reduced. By doing this, the intensity of the light received by the photodiode 12 is increased to enhance the image quality. In doing so, however, the generation of excessive parasitic capacitance between the metal lines is likely to cause problems such as leakage current, particularly in the peripheral driving part.